Millar is a graduate of the Sheffield Eagles Academy system[7] He made his professional debut for the Eagles on 26 June 2015 in a 14 points to 22 away triumph at London Broncos, he scored two consecutive tries in 5 minutes to put his team 10–6 in front just after the half-time break. Millar made his home debut against Hull Kingston Rovers in The Qualifiers at Bramall Lane. This time he scored a hat-trick in a 28–38 loss. Millar has forced his way into a regular first team spot ahead of the 2016 season, as one of the newest young talents to come out the Eagles' youth set-up. During that season he made eight starts and scored a total of 5 tries. The wing was now considered a permanent fixture in the Eagles line-up and throughout the course of the 2017 campaign he showed his versatility in playing as a fullback as well as his natural position on the wing. Millar played a total of 22 games (21 starts) during 2017, but his scoring run dried up as he only managed the one try against Oldham on the first day of the season. In 2019 he helped the Eagles to win the inaugural 1895 Cup as they defeated Widnes Vikings 36–18 in the final.[8] In 2021 it was announced Millar was set to leave the club at the end of the season.[9]
Millar joined Bradford Bulls ahead of the 2022 season on a two-year deal.[10]
On 12 December 2022 it was announced that Millar had signed for Widnes Vikings on a one-year deal.[11]
On 16 November 2023 it was reported that he had signed for Sheffield Eagles in the RFL Championship on a 2-year deal.[12]
